1. Field of the Invention
The present invention relates to the spectral response of matter to applied light, and more specifically, it relates to spectroscopic tissue characterization and imaging.
2. Description of Related Art
Detection and imaging of living tissue is a major objective in a variety of fields from biology and biophysics to biomedicine and clinical studies, enabling discovery of cellular function, screening for diseases, synthesis of new drugs and evaluation of treatment plans. Because of the microscopic size among the various cellular constituents, techniques such as confocal reflectance and confocal fluorescence spectroscopy are used, e.g., to measure the response of matter to applied light. Such response may include fluorescence, scattering, absorption and Raman scattering. In vivo imaging is particularly important because it enables real time feedback, and could greatly reduce the duration of exploratory or therapeutic procedures, and allows rapid optimization of design and treatment parameters.
There is a tremendous medical, technological and scientific need for a portable apparatus for rapid spectroscopic characterization and imaging of materials including tissues. Though a great deal of progress has occurred, the potential of this technology has not been fully explored due to various technological limitations including insufficient computer power, availability and cost of fast electronics and availability of suitable light sources. However, it is anticipated that continuous technological advances could address existing limitations in the near future. Generally, the response of tissue to the applied light occurs on a nanosecond timescale and is highly wavelength dependent. The presence of many different tissue types renders the response function very complicated. The ability to identify and map out tissue components or tissue constituents would be greatly enhanced if it were possible to simultaneously record the temporal and spectral response to exposure to light pulses at different wavelengths that are temporally short compared to the response of the tissue.
Currently, methods for spectroscopic analysis in real time (suitable for in-vivo applications) are performed either in the spectral domain, using a CW laser at a particular wavelength or in the time domain using a short (most often picosecond) pulse at a single wavelength. Tunable laser sources are limited in their bandwidth. Currently, pulsed laser radiation at multiple wavelengths is possible by either utilizing many distinct lasers simultaneously, which is prohibitively expensive and the availability of laser wavelengths is very limited and does not lead to portability, or by utilizing nonlinear conversion techniques, namely optical parametric oscillators. The second technique requires a long time (as long as 10s of seconds) to tune from one frequency to the next. This prevents in vivo analysis, since the time necessary to acquire data is prohibitively long. It is apparent that the field of spectroscopic characterization and imaging of material and tissues would greatly benefit by the availability of a laser source capable of simultaneously producing light (or laser) pulses at discrete wavelengths across a wide bandwidth.
Recently, several techniques efficiently producing wide bandwidth sources have been experimentally demonstrated. These techniques rely on the interaction of intense laser beams with a gas. Raman scattering occurs when a laser beam at frequency ωL excites a rotational or a vibrational molecular transition, at frequency ωR. The molecular motion modulates the applied laser beam and produces new sidebands at the sum and difference frequencies, (ωL−ωR, ωL+ωR, ωL−2ωR, ωL+2ωR, etc.). This process is typically inefficient and generates few sidebands. The Raman generation processes becomes highly efficient when two intense laser beams with an appropriately chosen frequency difference approximately equal to the Raman transition are applied simultaneously and collinearly. This produces a set of discrete frequencies that span over four octaves of bandwidths (from 200 nm to 3 μm). The pulse duration at each sideband is equal or shorter to that of the excitation laser pulses and can be in the range of 10−15 to 10−8 seconds. The sidebands are generated collinearly in a nearly TEM00 mode and are both spatially and temporally coherent.